This invention relates to a thermal overload protection apparatus for either a commutator or slipring motor.
One type of thermal overload protection is known from German published application No. 3,306,663 by Volkmar Koenig, published Feb. 2, 1984. In this known device used specifically to monitor the temperature of a hammer brush holder of an electrical motor, there is a thermal switch with a bimetallic or semiconductor element inserted in a side pocket-shaped seat of the brush mounting arm of a hammer brush holder, by which, in the event of excessive temperature rise and consequent exceeding of a specified temperature, a switch interrupts the electrical power supplied to the brush and thereby switches off the motor. It is desirable to provide a mechanism for thermal overload protection without the increase in cost and the decrease in reliability of having a thermally actuated electrical switch in series with the brushes.
The mechanism and application of the so-called form memory effect are known from existing applications (e.g., the journal "Material und Technik", 1978, No. 2, p. 59-65). Memory alloys are materials which after plastic deformation at low temperatures resume their original shape when heated above a critical temperature ("memory effect"). Memory elements having both a so-called one-way effect as welll as those with a so-called "two-way effect" are described in detail. With the one-way effect, the original shape is resumed only once when being heated, while with the two-way effect there is a reversible temperature-dependent form change behavior, wherein during the heating process the distortion is not reversed entirely and following subsequent cooling a form change toward the original distortion is attained. Both the one-way as well as the two-way effect can each be repeated as often as desirable.
By designing a brush holder in accordance with this invention it is possible to lift the brushes themselves, without additional power assistance means, from the surface of the commutator or the slipring suddenly and with great force and thereby to assure an interruption of the power circuit or switch off of the motor which is to be protected from excessive heatup. The expense for said overload protection can be further reduced for a hammer brush holder system of a commutator or a slipring motor by providing a memory element with a two-way effect. Special design, fabrication as well as manufacturing expense for additional bimetallic or semiconductor temperature switches and their mechanical mountings and electrical connections are thereby eliminated.
The object of this invention is to develop an overload protection which in contrast to the known device for temperature monitoring previously described, is easier to manufacture and install. A second object of this invention is to provide high operating reliability by maintaining a defined switch-off point when attaining or exceeding a preset temperature set value of the overload protection. A further object of this invention is to use the memory effect of certain alloys in the manufacture of cost effective, reliable thermal overload protection apparatus.